[Fedora-xen] TCP checksum corruption

[Mike McGrath]

Daniel P. Berrange wrote:
> On Tue, May 08, 2007 at 11:39:14AM -0500, Mike McGrath wrote:
>   
>> We're using xen heavily in Fedora's Infrastructure and presently a 
>> number of the xen domU hosts are experiencing terrible checksum issues.  
>> I've tried the ethtool -K eth0 tx off fix and it didn't work.
>>     
>
> What sort of network config have you got with these ?  Briding straight
> to physical device, or NAT'd ?
>
>   
Bridge
>
> There are a couple issues at play:
>
>  - There is a general bug in 2.6.20  that breaks checksum offload
>    when used with NAT.
>  - In 2.6.19 or later Dom0 will transmits to guests using checksum
>    offload, so DHCP client in the guest will mistakenly thing it
>    has a corrupt checksum.
>
> To address the first bug requires disabling checksum offload in the eth0 in
> the guest. ethtool -K eth0 tx off    in the guest should do it.
>
> To address the 2nd is really difficult since the FC6 install images themsves
> have a broken DHCP client for example, so we need to workaround it in the
> kernel. This can be done by disabling checksums on the device in Dom0 - any
> of vifN.0,  xenbr0, phet0 should have ethtook -K <dev> tx off done.
>
> NB, ignore eth0 in Dom0, that's a fake device so turning off tx on that does
> not fix things.
>
> So in summary, to get it working in general case requires:
>
>    ethtool -K eth0 tx off    in guest
>
> And
>
>    ethtool -K <dev> tx off   on whatever bridge device the guest is attached to
>   
I've actually run that on every interface on every dom[0,U] on the box 
:).  I've also tried it on two other hosts.  One a RHEL5 dom0 and the 
other had different hardware but was also a FC6 dom0.  I can arrange 
access to the box if you're interested.

    -Mike